A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC--DC converters are frequently employed to advantage. DC--DC converters generally include an inverter, a primary winding of a transformer coupled to the inverter, a rectifier coupled to a secondary winding of the transformer, and an output filter. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the converter. The output filter, typically an output inductor and an output capacitor, smooths and filters the output voltage for delivery to a load.
There are two preferable methods for regulating the output voltage of the converter, namely, voltage-mode control and current-mode control. In voltage-mode control, the output voltage of the converter is fed back through a compensated error amplifier to produce a threshold voltage. A comparator then generates a drive signal for the power switch as a function of the threshold voltage (e.g., the comparator may turn on the power switch when the threshold voltage exceeds a periodic ramp signal). In current-mode control, a current in the converter, such as a switch current or an inductor current, is substituted for, or added to, the periodic ramp signal. The output voltage of the converter is also fed back through the compensated error amplifier to provide the threshold voltage for the comparator. The aforementioned methods and variations thereof are widely used and are adequate for many loads.
Low voltage digital loads that generate wide bandwidth step changes in output current, however, may cause the converter to produce unacceptable transient output voltages. The variations in the output voltage are caused, in part, by the output inductor. While the output filter advantageously attenuates output ripple current resulting from the switching action of the converter, the output inductor impedes rapid changes in the output current of the converter. Designers have attempted to reduce the effects of the output inductor by increasing a bandwidth of the control loop or by decreasing the size of the output inductor. The bandwidth of the control loop, however, is limited because the control loop may not be able to reliably compensate for the interactions between the various reactive elements in the converter at high frequencies. The limited bandwidth of common control elements, such as operational amplifiers and optical isolators, as well as the need to preserve noise immunity in the control process, impose additional limitations. In general, it may be difficult to operate the control loop at a frequency greater than 10-15% of the switching frequency of the converter using readily available sensory and control elements. Reducing the size of the output inductor may also be of limited value, since any reduction in the size of output inductor may increase the ripple current in the output capacitor, resulting in increased output ripple voltage.
Since the output voltage of the converter should ideally be regulated to a substantially constant value, it is important that the output impedance of the converter be kept as low as possible to reduce any effect on the output voltage caused by changes in the output current. Further, the output impedance should be minimized to meet the output ripple voltage requirements, which may be less than 1% of the output voltage. The output impedance of the converter is generally a function of frequency components in the output current. When a substantial portion of the frequency components are well below the bandwidth of the converter control loop, a high-gain control loop may be able to adequately compensate for changes in the output current, resulting in a very low output impedance. As a substantial portion of the frequency components approach or exceed the bandwidth of the converter control loop, however, the control loop may become ineffective in compensating for the fast output current changes. The output impedance of the converter may be principally determined, at high frequencies, by the characteristics of the passive components of the output filter, such as the output capacitor. As frequency increases, the impedance of the output capacitor decreases until an equivalent series resistance or a reactance (due to the parasitic inductance) of the output capacitor becomes larger than the reactance of the output capacitor itself. The parasitic properties of the interconnecting wiring paths that couple the output capacitor to the circuit are typically included in determining the parasitic properties of the output capacitor. Thus, the interconnecting wiring path between the output capacitor and the load should be as short as possible to minimize the parasitic inductance of the output capacitor, thereby minimizing the output impedance of the converter.
A common solution to the aforementioned problem has been to add substantial amounts of capacitance at the output of the converter and at the load, thereby decreasing the output impedance of the converter. The additional capacitance, however, will increase both the size and cost of the converter. Another way to reduce the effects of fast transients in a converter is to interleave a number of power trains togther. Interleaved power trains may accommodate smaller output inductors without substantial increases in the total output ripple current. Multiple power trains, however, may significantly increase the complexity and cost of the converter.
Yet another way to reduce the effects of the transients at the output of the converter is to employ output current feedforward. For a discussion of conventional output current feedforward techniques for converters with current-mode control, see Near-Optimum Dynamic Regulation of DC--DC Converters using Feed-Forward of Output Current and Input Voltage with Current-Mode Control, IEEE Transactions on Power Electronics, by Richard Redl and Nathan Sokal, pp. 181-192, Vol. PE-1, No. 3, July 1986, which is herein incorporated by reference.
Output current feedforward techniques require that the output current of the converter be sensed. Circuit elements introduced into the converter circuitry to sense the output current, however, typically have an adverse effect on the output impedance of the converter, especially at high frequencies. In the aforementioned article, Redl and Sokal describe the use of a small current sensing transformer in series with the output capacitor to sense the current through the output capacitor. While the current sensing transformer may be effective in many applications, the introduction of any series element in the output circuit of the converter(on the load side of the output capacitors) may adversely affect the output impedance of the converter, especially at high frequencies. Converters using the approach described by Redl and Sokal, therefore, cannot be readily employed in many applications, particularly applications wherein the loads experience wide bandwidth current changes. Further, board space for the converter may be limited, requiring the current sensing transformer to be placed on the end user's board, presenting further obstacles.
Accordingly, what is needed in the art is a system and method that determines the output current of a power converter that overcomes the deficiencies of the prior art.